A payload refers to the fraction of mass considered as cargo in relation to the overall mass of a vehicle. For the term to make sense, the cargo has to be delivered from an origin to a destination. In spaceflight, origin and destination can be reference systems such as orbits, landing sites on celestial bodies, or the Kerbal Space Center.
Since the term cargo can comprise absolutely everything (including rockets), and even Kerbals (typical payload) have their own means of propulsion, the distinction between cargo and vehicle can be problematic. As all stock rockets except three haven't any engines, which are powerful enough, they can be described as payloads.